Atrial fibrillation (AF) is a cardiac disorder involving an irregular, and often ineffective, quiver-type beating of the heart's two upper chambers (the atria). In certain forms of AF, blood may not be pumped completely out of the atria, pool along the atrial walls, and eventually clot. If a blood clot in the atria leaves the heart and becomes lodged in a brain artery, stroke can result.
AF may be associated with significant morbidity and mortality, primarily due to the increased risk of ischemic stroke. Patients who suffer from this arrhythmia have about a 3% to 4% absolute risk of stroke per year, but this varies significantly based on individual clinical features. Antithrombotic therapy with the vitamin K antagonist warfarin may be highly effective in stroke prevention and in improving survival. However, it may be associated with life threatening hemorrhage and may require intensive dosage monitoring.
AF affects more than 2.2 million people in the United States, and the prevalence of AF increases with age. Approximately 4% of people over age 60 have experienced an episode of AF. AF can occur in healthy people, but more often is associated with an underlying condition such as coronary heart disease, hypertension, valvular heart disease, and rheumatic heart disease. AF may also develop after cardiac or pulmonary surgery.
Treatments for AF include medications to decrease blood clotting, medications to slow down rapid heart rate, electric shock to restore normal heart rhythm (cardioversion), pulmonary vein antrum isolation (PVAI), and use of pacemakers to regulate heart beat rhythm.
Normally, a mammalian heart beat comprises phases called “diastole,” in which the heart relaxes and fills with blood, and “systole,” in which the heart contracts and pumps out the blood. An electrical wavefront typically starts in the “sinoatrial” (SA) node of the atrium, spreads over the two atria, and leads to contraction of cardiac muscle. When such an electrical wavefront reaches the “atrioventricular” (AV) node, the wavefront is delayed, which allows the atria to finish contracting, moving blood from the atria to the ventricles.
From the AV node, the electrical wavefront spreads through the His-Purkinje system, which comprises fibers that form a specialized conduction system that quickly propagates the wavefront throughout the ventricles, resulting in ventricular contraction. Contraction of the ventricles pumps blood into the lungs and body. At the end of contracting, the ventricles relax and the process repeats.
An electrocardiogram (ECG) can be used to assess heart rhythm and disturbances therein by measuring electrical activities of the heart that are detectable at surfaces of the body. An ECG typically comprises a repeated pattern of three measured electrical waveform components of a heartbeat: the “P wave,” the “Q wave,” and the “T wave.” The P wave results from atrial depolarization, i.e., the wavefront generated as electrical impulses from the SA node spread throughout the atrial musculature. The Q wave occurs at the beginning of a “QRS complex,” but may not always be present. The T wave involves electrical recovery of the ventricles.
The P wave precedes the QRS complex, which occurs as a result of ventricular depolarization. The QRS complex, a large waveform, typically comprises three waves, the “Q wave,” the “R wave,” and the “S wave,” but not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any combination of these waves can be referred to as a QRS complex. The Q wave represents depolarization of the interventricular septum. The R wave is typically the first positive deflection, and the S wave is the negative deflection that follows the R wave. The time interval between two consecutive beats, the so-called “beat interval,” is often measured from the R-wave of one beat to the R-wave of the following beat, and the time between two consecutive R waves is called the RR interval. A “PR interval” comprises the time it takes an electrical impulse to travel from the atria through the AV node, bundle of His, and bundle branches to the Purkinje's fibers; and the PR interval extends from the beginning of the P wave to the beginning of the QRS complex.
The QRS complex is usually the dominant feature of an ECG. The P wave is much smaller than the QRS complex because the atria generate less electrical activity than the larger ventricles. Other components of an ECG include the “Q-T Interval,” which represents the time necessary for ventricular depolarization and repolarization, and extends from the beginning of the QRS complex to the end of a T wave. By analyzing patterns of an ECG, insights into the condition of the heart can be obtained.
In an ECG from a heart with normal rhythm, large QRS complexes are separated by a fairly flat signal, except for a small upright bump (the P wave) about 120-200 ms before the QRS complex. A P wave is conducted when atrial electrical activity conducts through the AV node, causing electrical activation of the ventricles and the QRS complex. At most one P wave in an RR interval is conducted, and any other P waves in the same RR interval are non-conducted. A P wave is non-conducted when it fails to lead to a QRS complex. Non-conducted P waves can result from a premature P wave, a condition called AV block, and other reasons. P waves non-conducted as a result of AV block are said to be blocked P waves.
In atrial flutter, the atrial rhythm can increase to approximately 250-350 beats per minute. Increased atrial rhythms are sometimes detected as continuous waves in an ECG, with several waves appearing in a continuous, connected pattern in each RR interval: a pattern substantially different from the normal pattern of a single P wave in each RR interval. Such waves of continuous, cyclic atrial activity are called flutter waves or F-waves, and may form a sawtooth pattern in an ECG. During atrial flutter, the ventricular response can become locked into a regular pattern with the atrial activity, so that, for instance, every third flutter wave results in a QRS complex while the other flutter waves are non-conducted. In other cases, conduction of the flutter waves can be more random, resulting in an irregular ventricular rhythm.
Rapid atrial rhythm rates, generally over 350-400 beats per minute, are called AF. Such atrial activity can be visible in the RR interval as continuous, cyclic activity referred to as “f waves,” or coarse AF. Typically, the f waves are cyclic, but not as organized or consistent in shape as the F waves of atrial flutter. When viewed in two ECG channels, the cyclic activity of f waves may be seen to alternate back and forth between channels in what appears to be modulated electrical activity. At other times, AF may be present with no obvious cyclic activity visible in an ECG, but with low amplitude disorganized “noise” in the baseline. In other cases, there may be total absence of atrial activity, suggesting that the AF has become disorganized.